public class ParentClass {

    int intField = 10;

    class InnerClass extends ParentClass {

    }

    public static void main(String... args) {
        ParentClass parentClass = new ParentClass();
        InnerClass innerClass = parentClass.new InnerClass();

        System.out.println(innerClass.intField);

        InnerClass innerClass2 = innerClass.new InnerClass();
    }
}

public class Test {

    public static void main(String[] args) {
        // whatever...
    }

    class TestChild extends Test {
        // whatever...
    }
}